An Introduction by A.G.Helyar

The majority of sundials which exist today can be broadly classified into horizontal or vertical types, although a very wide range of designs is possible. A horizontal sundial comprises a flat plate, usually made of brass or bronze, to which is fixed a second smaller perpendicular plate, often of a decorative design but roughly triangular in shape. The assembly is fixed on a pedestal in such a fashion as to cause the shadow of the vertical triangle to fall onto a series of radial lines marked on the base. These lines are drawn to represent the hours of the day and thus the position of the shadow indicates the time. A vertical sundial has it's hour lines marked on a vertical surface, such as the wall of a building, with the shadow being cast by a thin rod protruding from the wall at an angle.

The origin of the sundial is lost in the mists of antiquity, but the variation in the length and direction of the shadow cast by a tree or rock, as the sun moves across the sky, must have been noted very early in the history of man. The concept of dividing the day into parts came much later and the earliest sundial known, consisting of some crude scratchings on a stone fragment, is dated at about 1500 BC.

The shadow casting component of a sundial is known as the style or gnomon, and ancient sundials were based on a style consisting of a rod or pole placed at a right angle to the face of the dial. Although the period from sunrise to sunset was divided into twelve 'hours' (probably as a consequence of the year being divided into twelve months) the vertical style registered short duration hours in the winter, when the period of daylight was short, and long hours in the summer. This unequal hours system remained in use for many centuries until someone realised that if the style was inclined so as to be parallel to the earth's axis, equal hours would be registered.

In Britain, in Saxon times, the period between successive sun rises was divided into eight equal parts known as tides (we still talk of noontide and eventide) and sundials of this period are usually divided into four segments, although some are further sub-divided to give twelve parts indicating the influence of the Roman 24 hour system.

Sundials, if properly constructed, can indicate the time of day to within a minute or so and after the invention of mechanical timepieces in the thirteenth century sundials were used to check the accuracy of clocks and watches for a further five hundred years. Most of us have probably, at one time or another, checked the time indicated by a sundial against the reading of our quartz controlled, split lap timing, dual time, back lit, works for ever at the bottom of the sea, wrist watch and noting that there was no agreement in the times, have rejected the sundial as a useless curiosity. To do so is to probably misunderstand the sundial and to reject an instrument which was the major timekeeping device for at least ten and probably twenty centuries.

The movement of the shadow cast by the gnomon or style of a sundial is caused by the apparent movement of the sun across the sky from sunrise to sunset. The sun rises in the east so, relative to a fixed point, places to the east experience sunrise earlier and places to the west start the day later. Every schoolboy knows that when it is noon in London it is 7.00 am in New York. However, what he might not know is that according to the sun it is at the same instant 11.50 am in Bath. This relatively local variation in time was of little consequence prior to the invention of the telegraph and the advent of railway timetables but, in 1884 by international agreement, the world was divided into 24 time zones and places within each zone started to use a common time. Thus in Britain all clocks are set to Greenwich time and an un-corrected sundial in Bath is apparently 10 minutes slow.

There are more reasons why sundial, or sun time, varies from clock time. To start with the earth does not move at a constant speed during it's annual journey round the sun. This is because the earth's orbit is elliptical, with the sun at one focus, and consequently the earth is sometimes nearer the sun and sometimes further away. The effect of this, together with speed variations due to the tilt of the earth's axis, causes the sun to sometimes appear to be ahead of schedule and sometimes behind.

The combined effect of these speed variations is embodied in a table of correction factors known as the Equation of Time. This table, often plotted as a graph, gives the number of minutes to be added or subtracted from sundial time, for each day of the year, in order to arrive at clock time.

The equation of time remains practically constant from year to year (a slight variation occurs due to leap years) and it can be seen that on four days in the year, about April 16th., June 14th., September 2nd., and December 25th., no correction is required and the sun time will agree with clock time. The maximum deviation occurs on November 3rd./4th. when 16 minutes 23 seconds must be subtracted from the time given by a sundial.

A further discrepancy between sundial time and the time given by your watch can also occur if daylight saving time is in use. This is when, in the UK, we advance our clocks by one hour in the summer and use British Summer Time (BST) instead of Greenwich Mean Time (GMT).

To summarise then we have three reasons why our sundial appears to indicate the time incorrectly. Firstly because of it's distance from the local time meridian, secondly due to the necessity to apply the correction given by the equation of time, and finally, in summer months, because our clocks and watches are set to BST.

In Sussex we are fortunate in being so close to the Greenwich meridian, it runs from Peacehaven on the coast, through Danehill and East Grinstead, that we need not bother with any correction on this account. Well designed sundials further afield should have this longitude correction built in. However corrections due to the equation of time and BST must always be made and many sundials incorporate a graph of the equation of time to remind us.

There are numerous different types of sundials but the two most commonly seen are the horizontal type, usually placed on a short pedestal for convenient viewing, and the wall mounted vertical type. The horizontal sundial, often erected as the centrepiece in ornamental gardens, has the advantage that it will tell the time whenever the sun is shining, whereas other types due to their design or location, may only indicate during certain hours of the day. A disadvantage of the horizontal dial is that it cannot be read at a distance.

A horizontal sundial comprises a flat plate engraved with radial hour lines and supporting a triangular vertical fin, known as the gnomon, which casts a shadow over the hour lines to indicate the time of day. The hours indicated are usually from 6 am to 6 pm although dials calibrated from 5 am to 7 pm are not uncommon. The 6 am/6 pm hour lines always form a single straight line across the dial with the noon line perpendicular to it. The gnomon is usually mounted along the noon line and has it's slanting edge rising from the focus of the hour lines. The angle between the slanting edge, often referred to as the style, and the dial plate is equal to the latitude of the dial location and it is the shadow of this edge which actually indicates the time of day. The sundial is mounted on it's pedestal such that the gnomon lies directly along a north/south line.

When a sundial is constructed it is relatively easy to build in a correction to allow for the difference in sun time between the meridian of the sundial and the local time meridian. In a horizontal dial this correction has the effect of slightly off-setting the the noon line from the fixed north-south line of the gnomon. Of course all the other hour lines are also shifted and the effect is to slightly rotate the dial plate anticlockwise from the gnomon if the dial is situated to the west of the local time meridian, and clockwise if it is to the east.

Early horizontal sundials usually have their dial plates embellished with a host of additional lines, figures, and ornamentation. This additional marking is known as dial furniture and is designed to give further information to the expert reader. This extra information can include:-

In addition to all this there is nearly always a motto and the makers name and date and a dial carrying anything approaching this amount of information can be virtually unreadable.

The great advantage possessed by a vertical sundial is that it can be viewed from a distance. This type of dial comprises a vertical dial plate, marked with the hour lines, from which protrudes the style at an angle equal to the compliment of the angle of latitude (the co-latitude) of the location. In the case of a vertical dial a solid gnomon is not fitted so enabling a full view of the hour lines on the dial plate from any angle.

Vertical sundials are usually found on the walls of buildings. Older versions are normally wooden panels, with the hour lines painted black on a white background, but modern types are made in a variety of materials and often use the wall itself as the dial plate. When next in High Street UK the reader is enjoined to wrench his eyes away from the delights of the shop windows and to look up above the flashing neons and the shop signs and behold the facades of the buildings. Apart from the occasional vertical sundial (reward enough!) you will be surprised at the view.

As with a horizontal dial the 6 am/6 pm hour lines on a vertical dial form a straight line which, if the dial faces directly south, is horizontal and with which the vertical noon line forms a 'T'. The style emerges from the dial plate at the intersection of the 6-o'-clock and noon lines and, in the case of a direct south facing dial, in the same plane as the noon line.

The direct south facing vertical dial is symmetrical about it's vertical noon line and is the most useful of the vertical dials since it is capable of giving a reading from sunrise to sunset. However few walls face directly south and it is possible to design vertical dials to face in any direction. Apart from those facing directly north, south, east, or west, these dials divide into four categories, those facing north-east, north- west, south-east, or south-west. The angle the dial makes with north in the first two categories, or south in the last two, is known as it's declination and this whole group of sundials are known as vertical decliners. In addition to declining, vertical dials can also incline (lean forward) or recline (lean backward), each type requiring it's own specialised approach to the calculation of the hour angles.

aghelyar@pavilion.co.uk

Return to Home Page